This invention relates to a flywheel energy storage system, and more particularly to a flywheel energy storage system in which electrical energy is converted into mechanical energy by means of a dynamoelectric machine and stored in a flywheel, and in which the stored mechanical energy is taken out as electrical energy from the dynamoelectric machine.
A conventional storage system of this type is illustrated in FIG. 1. In the figure, a dynamoelectric machine 2 is connected to a flywheel 4 by a shaft 6 which is supported by a bearing 8. A magnetic bearing may be used with the bearing 8. An auxiliary machine 10 is connected to the dynamoelectric machine 2 and the bearing 8 in order to achieve smooth and continuous operation of the system. The auxiliary machine 10 typically includes a plurality of different auxiliary devices. The auxiliary machine 10 has further connected thereto a low voltage power source 12 for supplying power. Also, a mechanical brake 14 is connected to the shaft 6 for stopping the system.
With the above construction, when energy is to be stored, the dynamoelectric machine 2 is operated as an electric motor to accelerate the flywheel 4. Therefore, by inputting electric power, energy is stored as mechanical energy in the flywheel 4. On the contrary, when energy is to be taken out, the flywheel 4 is decelerated by operating the dynamoelectric machine 2 as an electric generator to release electric power to the load 18. Generally, the rotational speed of the flywheel 4 is controlled so as to be between an upper and a lower limit, and a control device 16 is inserted between power source/load 18 and the dynamoelectric machine 2. This kind of storage system is used as means for accommodating changes in load on the power line.
In order to smoothly continuously operate this storage system, certain operational tasks are required to be performed, e.g., the bearing 8 must be lubricated or electromagnetically excited, the dynamoelectric machine 2 must be cooled by air flow or other means, the environment around the flywheel 4 must be depressurized to decrease operating losses due to air resistance, feed oil must be supplied to the vacuum seal, etc. To accomplish these operational tasks associated with the operation of the dynamoelectric machine and the flywheel, a plurality of auxiliary devices are provided in an auxiliary machine 10 and an auxiliary power source is provided to power such auxiliary machine. The auxiliary power source for the auxiliary machine 10 is herein shown as electrical power from a low voltage power source 12.
Shut down of the storage system is required, for example, upon failure of the main power source and/or the auxiliary power source. Shut down of the storage system until a lower speed limit is reached can be achieved by releasing energy from the flywheel 4, but when the speed is reduced to a speed lower than the lower speed limit, the system is forced to a final shut down by dissipating the energy in a generating brake resistor within the control device or by operating the mechanical brake 14 to mechanically force the speed of the flywheel below the lower speed limit.
Since the conventional storage system is constructed as described above, when there is no surplus electrical power to be returned to the power source 18 when the storage system is in the process of being shut down, the shutting down must be achieved after the storage system is switched to the generating brake shut-down mode even below the lower speed limit, and upon power failure in the control circuit, the storage system must be shut down by the mechanical brake 14. However, since the mechanical brake 14 generally provides a braking torque by pressing a brake shoe against a drum or a disc, problems of dissipating heat and maintenance arise when the capacity of the flywheel 4 is large. Further, during power failure, the operation of the auxiliary machine 10 may often be seriously affected, providing a disadvantage that a large-capacity battery is needed. In the worst case, the storage system itself may be destroyed by the stoppage of the auxiliary machine 10.